Varactors have their uses, but this design is more versatile in many situations.
Electronically-variable capacitors, whether used for T&M or in an end circuit, usually have a maximum capacitance of a few hundred picofarads, and a limited adjustment range. This Design Idea demonstrates a wide-range, variable, high-valued capacitor.
Figure 1 The circuit in the dashed box illustrates the variable capacitor topology.
Here are the relevant equations:
Figure 2 Practical implementation of the variable capacitor. The Verr blocks are discussed later.
With the values shown in the schematic:
ki = 1
kd = C1 × (P1+R7)
Changing the value of the potentiometer we get a capacitance of 100 nF to 4.8 µF.
Figure 3 Simulation results for P1 = 3.2 kΩ.
Figure 4 Actual waveforms of Ic and Vc.
As a further test, I connected a 2.2 mH coil between Vc and ground. The circuit rings at 1.87 kHz, which agrees well with the expectation.
Figure 5 Ringing (1.87 kHz) of 2.2 mH and 3.3 µF.
Influence of op-amp common-mode rejection
Considering the error sources at the outputs of U1 and U3 due to the mismatches in R3-R6 and R8-R11, we get:
We can neglect the second term due to very high value of denominator.
The CMRR due to resistor mismatch is:
where Gd is the differential gain.
Using 0.1% resistors for R3-R6, the CMRR will be 54 dB.
—Download TINA simulation files.
—Gheorghe Plasoianu has a Masters degree in electrical engineering from the Polytechnics Institute of Bucharest.
- Linear Technology, Design Note 1023: Precision Matched Resistors Automatically Improve Differential Amplifier CMRR – Here’s How
- A guide to using FETs for voltage controlled circuits
- Sensing current on the high side
- Dramatically increase the frequency range of RC-based voltage-controlled oscillators
- Light-controlled oscillator uses solar cell junction capacitance
- Circuit simulates a loudspeaker's impedance curve
- Smart ripple canceller offers near-zero dropout
- Submit a Design Idea